Abstract: A main ballasting weight (10) straight from the foundry for a farming tractor, comprising upper faces and lower faces (11, 12), two faces (13, 14) oriented transversely to the direction of movement of the tractor, and two lateral faces (15, 16) parallel to the direction of movement of the tractor, said lower face comprising dovetail type means of connection (121, 122) to provide an anchoring with a secondary weight, said upper face forming an upper plate comprising a means of pre-positioning and centring (110) of a box (20) designed to be mounted on said main weight, said lateral faces (15, 16) being able to cooperate with lateral flanges (22, 23) of said box which are attached bearing against said lateral faces and having fixation orifices (150) able to receive fixation screws cooperating with said lateral flanges for the fixation of said box to said main weight.